Ubuntu guest power-down dialog potentially invisible in seamless mode

In a Heron install, clicking on the power button in the system tray dims the screen and displays a power-down dialog.

In seamless mode, only the parts of this dialog that are in front of a window are drawn. The rest of the dialog is invisible.
If there are no windows that overlap, the entire dialog is invisible. Key presses work OK, as escape still cancels it.